A complex adaptive system would be the spread of the Zika Virus. The agents would be a mosquitos (healthy and infected), and a healthy population of people - a few men, women, and a subset of pregnant women. The environment is an area that is warm and moist.
All of the agents move around randomly. When the mosquito comes in contact with a person they will have the virus. Another interaction rule would be to assign certain agents to be pairs (male/female) and if one of the pairs gets the virus the other would automatically get the virus based on sexual contact.